Method for expanding the uniformly shielded area in a short-length open-ended cylindrical magnetic shield

A compensation method is proposed by which the uniformly shielded area of t he axial magnetic field in a relatively short, open-structure axial magneti c shield can be extended. An open-ended cylindrical magnetic shield of 120 cm in length, 52 cm inner diameter, and similar to 0.5 mm total thickness o f the shielding material is used to demonstrate the idea. The shield axis i s oriented along the horizontal component (similar to 320 mG) of the Earth' s magnetic field. A simple way to increase the axial shielding factor is to use a pair of compensating coaxial ring coils set at both open ends of the shield. This increases, however, the radial gradient of the shielded field since the axial compensation field is stronger towards the shield axis. In order to decrease the radial gradient, an additional ring coil is wound ar ound the middle part of the outer surface of the shield. The compensating f ield generated by this central ring coil is stronger towards the inner surf ace of the shield, and it helps, therefore, to unify the axial resultant fi eld over a wider area inside the shield. The axial shielding factor obtaine d with this compensation according to the proposed method is 128, in contra st to only 16.4 obtained with compensation by a set of two ring coils. The field gradients observed are 1.2 mu G/cm along the length direction and 2.7 mu G/cm along the radial direction, in contrast to the 14 mu G/cm axial an d 78 mu G/cm radial gradients obtained with compensation by a set of two ri ng coils. (C) 1999 American Institute of Physics. [S0021-8979(99)55508-4].